Conventionally, many of parts, such as chassis parts and body structural parts of automobiles, have been manufactured by press forming steel sheets having certain strength. In recent years, from the viewpoint of global environmental conservation, weight reduction of automotive bodies has been strongly desired, and efforts have been continuously made to increase the strength of steel sheets to be used and to reduce their thickness. However, since press formability of steel sheets decreases as the strength of the sheet steels increases, many of the sheet steels are becoming more difficult to be formed into desired shapes.
Accordingly, Patent Literature 1 proposes a forming technique called hot pressing, which enables both of simplification of forming and strengthening by quenching a steel sheet simultaneously with forming of the heated steel sheet using a mold constituted of a die and a punch. However, as for this hot pressing, since the steel sheet is heated to a high temperature of around 950° C. before the hot pressing, scales (iron oxide) are formed on a surface of the steel sheet. Accordingly, the scales formed on the surface of the steel sheet, by being peeled off upon the hot pressing, damage the mold or damage the surface of the part after the hot pressing. Further, the scales remaining on the surface of the part cause defects in appearance and decrease in paint adhesion. Accordingly, the scales on the surface of the part are usually removed by performing a treatment such as pickling or shot blasting, but this makes the manufacturing process complicated and results in decrease in productivity. Further, parts such as chassis parts or body structural parts of automobiles also require excellent corrosion resistance, but anti-rust coating such as a plating layer is not provided on a hot-pressed part manufactured by the above-mentioned process and hence, the corrosion resistance is very insufficient.
In view of such background, a hot pressing technique capable of suppressing formation of scales upon heating before hot pressing and improving corrosion resistance of parts after the hot pressing is being demanded, and a steel sheet provided with a coating such as a plating layer on a surface thereof and a method of hot pressing using it have been proposed. For example, Patent Literature 2 discloses a method of manufacturing a hot-pressed part having excellent corrosion resistance, in which a steel sheet coated with Zn or Zn-based alloy is hot pressed and a Zn—Fe-based compound or a Zn—Fe—Al-based compound is provided on a surface thereof. Further, Patent Literature 3 discloses a hot-pressed product (part) excellent in workability, weldability, and corrosion resistance, which is formed by heating at 700° C. to 1000° C. for 2 to 20 minutes and thereafter hot pressing a galvannealed steel sheet, and providing a plating layer including a Fe—Zn solid solution phase on a surface thereof.